A conventional high voltage aluminum electrolytic capacitor includes an anode foil having an aluminum foil expanded on the effective surface area by an etching process and an oxide film formed on the surface of the aluminum foil. A capacitor element is composed by winding this anode foil, a cathode foil and a separator between the two foils. By impregnating the capacitor element with a driving electrolyte solution, and sealing the capacitor element in a case, a high voltage aluminum electrolytic capacitor is manufactured.
The oxide film is formed in a forming process, and functions as a dielectric. The forming process, which forms an oxide film on the surface of the aluminum foil to create a high voltage aluminum electrolytic capacitor, is conducted according to the following procedure. Aluminum foil, roughened by an etching process, is boiled in purified water. Consequently, the aluminum foil is placed in an aqueous solution of boric acid, phosphoric acid or a salt thereof, and held at a constant current until reaching the forming voltage. A first forming of an oxide film on the aluminum foil is thus performed, upon reaching the forming voltage, by maintaining a constant voltage for a specific time. Since voids are present inside the formed oxide film, the oxide film is in an unstable state. To remove voids, the formed aluminum foil is depolarized, and an additional layer of oxide film is formed by the above forming process. This process is generally repeated two or three times.
In the forming process, when forming at high voltage using a solution of organic acid as the forming solution, a forming solution of an extremely low concentration must be used in order to prevent discharge thereof. However, since the concentration control of the forming solution is difficult, among other reasons, a forming solution of organic acid is not used. Therefore, hitherto, in order that discharge might not occur if a forming solution of high concentration is required, boric acid, phosphoric acid or salts thereof are used.
When forming solutions of high concentration are used, however, since the aluminum foil is dissolved, or crystallization of oxide film is not promoted, the electrostatic capacity tends to be lower. On the other hand, diammonium adipate is conventionally used as the forming solution of an aluminum foil for low voltage aluminum electrolytic capacitors. If diammonium adipate is used as the forming solution of an aluminum foil for high voltage aluminum electrolytic capacitors, a more crystallized oxide film is formed as compared with the case of using boric acid, phosphoric acid or salts thereof as the forming solution. Although use of diammonium adipate heightens the electrostatic capacity of the electrolytic capacitor, defects in the oxide film are increased. Therefore, the leak current increases, defects in the oxide film are exposed while holding the voltage at the forming voltage in the forming process, and the voltage fluctuates. As a result, stable production of the electrolytic capacitor is impaired.
The present invention hence presents a method of manufacturing aluminum foil for a high voltage electrolytic capacitor having properties of high electrostatic capacity and small leak current.